A boiler is an apparatus in which water or some other aqueous temperature control liquid to which makeup water is added and from which blowdown is removed is vaporized into steam by the application of heat from a furnace or heat-generating process system. In most instances, the temperature control liquid is brought into close, indirect contact with the process system to facilitate heat transfer. Leakage in a boiler can result not only in contamination and fouling of the temperature control liquid and the process system, but also in undesired physical reactions. This is particularly true for the black liquor recovery boilers used in many paper mills. In black liquor recovery boilers, the escape or leakage of aqueous temperature control liquid from the so-called "water side" of the boiler into the hot, highly caustic "fire side" can result in violent explosions.
The prior art provides numerous techniques for monitoring and controlling leaks in black liquor recovery boilers and other boiler systems. These methods use various techniques, most notably two classes of mass balance methods, chemical tracer and water mass balances. For example, U.S. Pat. No. 5,363,693, Nevruz, teaches methods and apparatus for detecting leakage from recovery boiler systems using mass balance of water flows into and out of the boiler. The method then calculates the long and short term statistics for the drum balance of mass flow. From these calculations a t-test function is calculated to see if both long term and short term moving average of drum balances are significantly different, which in turn indicates whether a boiler leak is occurring. While water mass balance leak detection systems can be effective, as described in U.S. Pat. No. 5,663,489 (Thungstrom et al.), they are generally less sensitive than chemical tracer systems and do not discriminate between leaks in critical and non-critical portions of the boiler.
U.S. Pat. No. 5,320,967 (Avallone et al.) is an example of the other mass balance method, namely chemical tracers. Avallone discloses a boiler system leak detection method that involves introducing an inert tracer to the boiler in a known and uniform proportion to the feedwater, sensing a characteristic of the tracer in the boiler at steady state, converting the sensed characteristic to a value equivalent to the concentration of the tracer in the temperature control liquid, and activating a signal when there is excessive variance in the concentration of the tracer. However, the method disclosed by Avallone et al. is limited by its requirement that the tracer be detected (sensed) when the boiler is at steady state, which is said to occur only when there is no significant change in any of five process parameters: the concentration of the tracer in the boiler; the blowdown rate; the feedwater rate; the rate of feeding tracer to the boiler; and the steam rate in the absence of boiler leakage.
U.S. Pat. No. 5,565,619, Thungstrom et al. teaches methods and apparatus for monitoring boilers for leaks. The methods utilize a tracer compound which is added to the boiler water at a rate that is proportional to the blowdown water rate. The expected concentration of the tracer exiting the boiler is calculated utilizing non-equilibrium condition variables and compared to the actual concentration of tracer in the blowdown. If there is a statistically significant difference between actual and expected concentrations, a leak condition is indicated.
While this chemical tracer method is an improvement over Avallone in that the boiler does not need to be at steady-state in order to detect leaks, two issues have been discovered related to this method. First, this method assumes that boiler water mass is constant as steam load changes. With very small leaks, changes in boiler water mass with load changes can lead to false alarms due to changes in the tracer concentration. Second, it is difficult to apply statistical methods to the output of the method.
Consequently, there remains a need in the art for a chemical tracer-based leak detection system that does not assume that boiler water mass is constant and can operate with an automatic water level controller. Further, there is a need for a chemical tracer-based leak detection system whose output can be used with statistical methods.